Belgium is planning to build a doughnut-shaped island in the North Sea that will store wind energy by pumping water out of a hollow in the middle, as it looks for ways to lessen its reliance on nuclear power.

Those are fascinating plans. Excess power to pump the water out, water let back in runs turbines to create power when needed. I wonder what environmental impact this might have, being not too far off the coastline. I wish they'd given the dimension of the "island."

'tis strange I should be old and neither wise nor valiant. From "The Maid's Tragedy" by Beaumont & Fletcher

According to a Belgian newspaper it would be in the shape of a horseshoe and "slightly more than 2 km wide". Of course there's no concrete plan as yet so the dimension and location might change (and there is a real chance it won't go head at all, politics and especially Belgian politics, being what they are).

This one has a head of 374 meters (elevation difference between upper and lower reservoirs, and an area of about 7 hectares, and the resulting system can generate 330 MW of power and store 1,300 MWh of energy. That's comparable to the power you get from a smallish coal plant...

With an island, the head would be something like maybe 10 meters at most, I would think. Or less, even, so the area would have to be proportionally greater...

It's a peculiar idea. Similar ideas were drawn up for the Dutch and the Danes too, under the name "energy islands".

It's peculiar because it's a lot of investment for a little bit of balancing capacity. And in the case of Belgium, Netherlands and Denmark, they already have lots of cheap balancing, because they're each well-connected to their neighbours, and several of their neighbours are much bigger than they are, so the balancing requirement is relatively small.

Integrating storage with offshore wind might make sense if transmission costs to shore were a high proportion of system costs, and if they were (more than) proportional to their capacity. Transmission costs can be expensive, but this proposed island is only 3 or 4 km offshore, and the construction costs of the island must be huge by comparison.